Influence Of Isomorphous Substitution On The Reducing Capability Of Magnetite Coupled With Aqueous Fe(Ⅱ) And The Related Mechanisms

As the second abundant metal on Earth’s crust,iron is one of the most widespread and redox active metals.Iron commonly exists as iron（oxyhydr）oxide minerals.Among the iron（oxyhydr）oxides,magnetite is both a primary and secondary mineral with stable structure and cannot easily transform into other iron oxides.It is a ubiquitous environmental constituent and can be found in weathered clays and soils,in atmospheric aerosols,and in deposited marine and freshwater sediments.Surface octahedral Fe²⁺in magnetite can reduce several environmental contaminants.As the reduction reaction proceeds,magnetite is gradually oxidized and losses its reducing capability.Interestingly,in anaerobic environment,impelled by dissimilatory iron reducing bacteria（DIRB）,aqueous Fe（Ⅱ）is continuously generated from iron（oxyhydr）oxides.The adsorbed aqueous Fe（Ⅱ）can reduce octahedral Fe³⁺in the underlying magnetite to tetrahedral Fe²⁺and,thus,improves the structural Fe²⁺content of magnetite and effectively recover its reducing capability.In natural environments,the iron ions in natural magnetite are extensively isomorphously substituted by other cations.For example,Co-substituted magnetite(Fe_3-xCo_xO₄)is ubiquitous in soil and sediment.Impelled by iron reducing bacteria,Zn²⁺can be introduced into magnetite structure through the reduction of iron compounds.These isomorphous substitutions remarkably change the surface physiochemical properties and microstructure of magnetite,which probably affect the reducing capability of magnetite coupled with Fe（Ⅱ）.（2-chloro）nitrobenzene and Cr（Ⅵ）were chosen as target contaminants.The reductive removal of（2-chloro）nitrobenzene and Cr（Ⅵ）by substituted magnetite coupled with aqueous Fe（Ⅱ）was conducted.The surface and structural properties of substituted magnetite before and after reaction was characterized.The specific aims of this study were to（i）analyze the effect of isomorphous substitution on the physicochemistry properties and microstructure of magnetite,（ii）investigate the influence of substitution on the reducing capability of magnetite coupled with Fe（Ⅱ）,（iii）explore the changes of chemical compositon and mineral phrase of substituted magnetite during the reaction,and（iv）establish the relationship between the structure and reactivity of substituted magnetite.The main results are as follows:（1）Restriction mechanism of Zn substitution on the reducing capability of magnetite coupled with aqueous Fe（Ⅱ）.In neutral condition,nitrobenzene（NB）was neither adsorbed nor reduced by Zn-substituted magnetite（Fe3-xZnxO4）.Compared with aqueous Fe（Ⅱ）,magnetite coupled with Fe（Ⅱ）showed greater reducibility towards NB.Nitrobenzene（NB）was reduced to nitrosobenzene,hydroxylamine,and aniline.Various factors,such as aqueous Fe（Ⅱ）concentration,magnetite stoichiometry and Zn level,were investigated to illustrate their effects on the reduction processes.Higher Fe（Ⅱ）concentration and structural Fe²⁺content facilitated the reduction.Zn substitution generally improved the reduction activity of magnetite coupled with Fe（Ⅱ）via the improvement of electrical conductivity of magnetite.At lower Zn level,Zn²⁺primarily occupied the tetrahedral sites and substituted tetrahedral Fe³⁺,accompanied by Fe²⁺oxidation to Fe³⁺on octahedral sites.This process promotes electron transfer in the octahedral sites in magnetite and,thus,improved the electrical conductivity of magnetite and the reducibility of the coupled system.But at higher Zn content,a portion of Zn²⁺moved to the octahedral sites,which decreased relative amount of Fe in octahedral sites and decreased slightly the reducibility of the coupled system.（2）Microscopic mechanism of the effect of Co substitution on the reducing capability of magnetite couple with Fe（Ⅱ）.2-chloronitrobenzene（2-Cl-NB）was reduced to 2-chlorohydroxylamine and2-chloroaniline by Co-substituted magnetite(Fe_3-xCo_xO₄)coupled with Fe（Ⅱ）.Both the reaction rate constant kobs and extent of electron transfer illustrated that appropriate Co substitution promoted the reduction activity of the coupled systems,while excess Co retarded the process.A good linear correlation（R²≥0.94）between the kobs values and the electrical conductivity of Fe_3-xCo_xO₄ was established,revealing that Co substitution accelerated the electron transfer among aqueous Fe（Ⅱ）-magnetite-2-Cl-NB and,thus,promoted the reducibility of the coupled system.Cobalt primarily occupied the octahedral sites in magnetite,the redox pairs Co²⁺/Co³⁺and Fe²⁺/Fe³⁺on the octahedral sites accelerated the electron transfer in magnetite and accordingly improved the electrical conductivity of magnetite and the reducing activity of the coupled system.As Co substitution increased up to 0.85,however,structural Fe²⁺occupying the octahedral sites of magnetite was too low,resulting in a decrease in the reducing capability of the coupled system.During the redox reaction,the surface octahedral Fe²⁺and adsorbed Fe（Ⅱ）were gradually oxidized,while the spinel structure of Fe_3-xCo_xO₄ was maintained.（3）Reaction mechanism of the adsorption-reduction of Cr（Ⅵ）by Co-substituted magnetite coupled with aqueous Fe（Ⅱ）.Cr（Ⅵ）was found first adsorbed on the Fe_3-xCo_xO₄ surface and then reduced to Cr（Ⅱ）by the structural octahedral Fe²⁺and the absorbed Fe（Ⅱ）;the generated Cr（Ⅱ）was chelated with OH groupes on the Fe_3-xCo_xO₄ surface,or substituted octahedral Fe in Fe3-xCox O4.No visible variations of Cr（Ⅵ）adsorption by magnetite with different Co content were observed.Both the reaction kinetics and the electron efficiency revealed that Co substitution significantly improved the reactivity of Fe_3-xCo_xO₄/Fe（Ⅱ）towards Cr（Ⅵ）reduction,while an overdose of Co retarded the process,which was similar with the effect of Co substitution on the reducing capability of magnetite/Fe（Ⅱ）towards 2-Cl-NB.During Cr（Ⅵ）reduction,the bulk Fe²⁺and adsorbed Fe（Ⅱ）in Fe_3-xCo_xO₄ was oxidized without phase transformation.（4）Distinct effects of typical isomorphous substitution on the reducing capability of magnetite coupled with aqueous Fe（Ⅱ）and its related mechanism.Co,Mn,Zn,and Mg substitutions significantly improved the reduction activity of magnetite coupled with Fe（Ⅱ）,while Cr and Al decreased the reducibility of the coupled system.The reaction rate kobs was positively correlated to the adsorbed Fe（Ⅱ）level and electrical conductivity of substituted magnetite.The adsorption capacity of Fe（Ⅱ）and the electrical conductivity were the controlling factor of the reductive performance of the coupled system,which were related to the surface physiochemical properties（e.g.active site density）and microstructure（e.g.,state and occupancy）of magnetite.Co,Mn,Zn,Mg substitution remarkably increased the active site density of magnetite and improved the adsorption of Fe（Ⅱ）,which promoted the reducing capability of the coupled system,while Cr and Al showed the opposite.In the meanwhile,Mn and Co with thermodynamically favorable redox pairs,i.e.,Co²⁺/Co³⁺and Mn²⁺/Mn³⁺could promote rapid electron exchange with octahedral Fe²⁺/Fe³⁺,which improved the electrical conductivity of magnetite and accelerated the electron transfer among aqueous Fe（Ⅱ）-magnetite-nitrobenzene.The tetrahedral Zn²⁺induced that the oxidation of Fe²⁺to Fe³⁺on octahedral sites,which also promoted the electron transfer during the reduction.However,the octahedral Cr³⁺and Al³⁺decreased the octahedral Fe³⁺content of magnetite,which inhibited the electron transfer between Fe²⁺and Fe³⁺,and thus the reducibility of the coupled system.The above-mentioned results obtained in this study will not only be helpful to further illuminate the role of magnetite and its impact on the immobilization,transfer,transformation,and degradation of contaminants in anoxic environments,but also provide the theoretical basis for the application of magnetite in environmental management and remediation.